Another, more frequent cause for false positives was the presence of an
emission component probably originating from the sky background that has not
been fully cancelled-out by the sky-subtraction procedure. This may happen
e.g. when the target star is located in a region of the sky affected by
diffuse background emission, like emission nebulae and HII regions. Such
cases are easy to recognize because the H\(\alpha \)  and H\(\beta \)  emission
lines are (\(a\) ) typically quite sharp, single-component and symmetric, (\(b\) )
frequently accompanied by \([\) NII\(]\)  6548, 6584 nebular emission lines (which
do not form in the high density environment of an accretion disk), and (\(c\) )
usually displaced in velocity with respect to the M-giant spectrum by an
amount larger than expected from orbital motion in a symbiotic binary. In
multi-fibre spectroscopic surveys like GALAH, only a limited number of
fibres are assigned to record the sky background. They are distributed
over the field of view away from the position of known stars. This is
perfectly fine for a great fraction of the sky away from the Galactic plane.
There are however, here and there, regions of the sky affected by background
emission due to diffuse nebulosity. The intensity of such emission may vary
appreciably over limited angular distances, and the same may be the case for
the internal nebular gas dynamics and therefore the resulting wavelength for
H\(\alpha \)  and H\(\beta \)  emitted lines. Any sparse mapping realized by a
limited number of sky fibres of such a complex 2D pattern can naturally
result in non-null residuals of the sky-background subtraction, in addition
to other caveats of sky subtraction as explained in
[1]}.
